Plasmodium gametocytes are normally activated to complete their development to gametes in the gut of a vector mosquito. Activation appears to be a response to a specific chemical cue within the insect gut. The identity of this factor will be determined using preparations derived from Anopheles stephensi. The intracellular events that comprise activation and gamete formation have been inferred from studies with selected pharmacological agents. Direct measurements of intracellular pH and cyclic nucleotide concentrations will be performed on purified preparations of non-activated gametocytes. The proposed purification technique entails release of parasites from host red cells by immune lysis followed by separation on an Hypaque-Ficoll step gradient. Such preparations will also provide synchronous populations of developing cells for ultrastructural studies of gamete formation.